Data networks, particularly IEEE 802.11 wireless data networks, have become an expected or necessary resource in an increasing number of situations. Once present only in the workplace environment, these data networks are becoming ubiquitous. They are encountered in schools, libraries, retail environments, and other gathering locations. As the use of these networks moves from a business setting to a consumer oriented setting, the static web pages of yesterday are being overwhelmed by the mixing of media streams, particularly with the addition of video streams. While the solutions have evolved for handling video distribution, particularly using multicast, many of these solutions were developed for wired networks, and do not translate adequately to the wireless environment.
Wired (IEEE 802.3) and wireless (IEEE 802.11) networks implement two data transfer models: unicast and multicast. In the unicast model, packets are sent from a source to a particular single destination. Unicast is a one-to-one data transmission. Unicast protocols, such as those based on Transmission Control Protocol (TCP), provide for error detection and packet retransmission to recover from missing or damaged packets arriving at the destination.
In contrast, the multicast model allows for one-to-many distribution of data. Multicast uses User Datagram Protocol (UDP), which is a connectionless protocol that does not support retransmission of packets. Multicast distribution of video streams allows for a single stream including a single set of packets to be sent to a plurality of devices. For example, in a University lecture hall setting, multiple devices such as student laptops or tablets may subscribe to a single multicast video stream of the lecture. Multicast allows multiple users (e.g., students) to be supported by transmitting one multicast stream of packets (e.g., a video stream of the lecture). In contrast, unicast requires multiple individual sets of packets (e.g., multiple identical video streams of the lecture) to be sent to each of the users (e.g., each student). A disadvantage of multicast is that it does not support retransmission. Accordingly, the client devices must use other strategies to overcome dropped, damaged, or out-of-order multicast packets.
In the current systems, where a wireless access point is sending multicast transmissions to a set of wireless clients, the data rate of the multicast transmissions are being decreased in order to attempt to increase the reliability (or probability of reception) of multicast transmissions. While using a lower data rate for multicast increases reliability, it also dramatically increases the time required to transmit a data stream. For instance, a multicast stream may be transmitted at an 11 Megabyte per second (Mb/sec) data rate while the full high-throughput 802.11 unicast connection is capable of delivering 450 Mb/sec data rates.
Current adaptations of the multicast systems include (i) increasing the multicast transmission rate and (ii) converting the single multicast stream to a plurality of unicast streams. Both adaptations present disadvantages. In the first adaptation, increasing the multicast transmission rate may decrease required transmission times, but it also increases data loss for client devices that are located farther from the access point or for client devices that are dealing with compromised signal conditions. Given that there are no automatic retries or retransmission in multicast, this adaptation results in increased data loss which leads to stuttering video, drop-outs, etc. In the second adaptation, the conversion of a single multicast stream targeting multiple client devices into multiple unicast streams, each being addressed to a single client, results in an increased transmission time and this adaptation cannot accommodate a large number of client devices.